1. Field of the Invention
This invention relates to a method for preparing a magnetic recording medium.
2. Related Background Art
In recent years, higher densification of magnetic recording medium such as tape, disc and so forth has been more strongly demanded, and researches and developments of various mediums are under progress. The method of using a thin metal film for the magnetic recording layer of a magnetic recording medium is on of them.
As the method for forming such a metal thin film recording layer, studies have been made regarding the wet process by plating and physical vapor deposition methods, by way of vacuum vapor deposition, sputtering and so forth. However, since in the wet process, management of a plating bath is cumbersome and unsuitable for continuous film formation, and also severe management is required in waste liquor disposal, thus involving difficult problems, the method by use of physical vapor deposition method has recently become the main stream.
As the method by physical vapor deposition, the method of forming a recording layer by oblique vapor deposition of Co Ni alloy has been widely studied, and otherwise the method of forming a Co type alloy (primarily Co-Cr alloy) as the recording layer has been studied. A Co type alloy is excellent as the recording layer of a vertically magnetized film and can accomplish high density recording.
In formation of the recording layer according to the physical vapor deposition method such as vacuum vapor deposition or sputtering as described above, in most cases, it is necessary to form a film of the magnetic recording later while heating a non-magnetic substrate in order to obtain desired magnetic characteristics. By heating, rigidity of the substrate will be remarkably lowered. For this reason, the substrate can be conveyed with difficulty internally of the vacuum film formation device, whereby defects such as wrinkles will be readily generated on the substrate. Also, at the portion where the magnetic recording layer is formed, energy carried by the incident flux of particles is applied to elevate further the temperature of the substrate, whereby particularly wrinkles are readily generated. This tendency becomes further pronounced as the thickness of the substrate is reduced. The wrinkles formed on the substrate will affect greatly the magnetic characterics such as dropout, etc.
A vertically magnetized film has been generally deemed to be suitable for digital recording, and in that case, for accomplishing high density recording, it has been considered that the magnetic characteristics macroscopically observed are required to have strong vertical anisotropy (this corresponds to small diversion of readily magnetizable axes of individual microcrystals of a magnetic material from the medium normal line direction, when microscopically observed). For this reason, in order to orient the C-axis of the h.c.p. structure of the Co type alloy well in the normal line direction of the substrate, the incident direction of the flux of particles is restricted so that the incident flux of particles may be incident vertically onto the substrate during film formation, particularly at the initial stage of growth of the alloy film. However, when the flux of particles is restricted in this manner, film formation begins from the greatest density of flux of particles, which may be also a cause for promoting generation of wrinkles as described above.
Also, another problem arises when such vertically magnetized film is used for analog recording of broad band, such as VTR. More specifically, in a magnetic recording medium with strong vertical anisotropy, the reproduction outputs of some low frequency components are reduced, causing inconveniences in recording which requires signals of broad band. For example, in VTR, tracking signals or color difference signals and so forth employ a band of 1 MHz or lower and, when the output is lowered in this frequency band, defective tracking or "turbidity" of color will occur.
On the other hand, when the magnetic recording medium of the metal thin film is directly slid against a head, the partner will be excessively abraded and therefore it cannot be provided for practical application as such. For this reason, by providing a protective layer such as Co type oxide or SiO.sub.2 on the recording layer and further providing a lubricating layer comprising a fluorine type resin, etc. thereon, improvement of durability has been effected. However, depending on the use, durability is still insufficient, and in addition to a protective layer and a lubricating layer, it has been also required to investigate the mechanical strength of the magnetic recording layer itself.